Abstract: Aberrant activation of TAL1 oncogene is associated with up to 60% of T-cell acute lymphoblastic leukemia (T-ALL) patients. Its ectopic expression also led to development of leukemia or lymphoma in mice. In contrast, deletion of TAL1 in T-ALL cells lost leukemic phenotype and induced apoptosis. Furthermore, the TAL1 expressing T-ALL subtype is associated with poor prognosis and high rate of relapse. These data suggest that dysregulation of TAL1 oncogene plays an important role in T-ALL leukemogenesis. However, in normal hematopoiesis, TAL1 is a hematopoietic-specific member of the basic helix-loop-helix family of transcription factors required for self-renewal of hematopoietic stem cells and the development of all hematopoietic lineages. Because of its relevance to normal hematopoietic differentiation and T-cell leukemia, it is critical to know how TAL1 oncogene is differentially activated in normal hematopoietic cells and T-cell acute leukemia. Understanding of the epigenetic mechanisms governing TAL1 transcriptional regulation will provide a new insight into epigenetic control of hematopoiesis as well as pathogenesis of T-ALL diseases which may lead to new strategies for leukemia diagnosis and therapeutic approaches. We recently found that TAL1 transcription is regulated by different intra- and interchromosomal loops in normal hematopoietic and leukemia cells, respectively. These intra- and interchromosomal loops alter the cell- type specific enhancers that interact with the TAL1 promoter. Based on these data, we hypothesize that repositioning of the TAL1 gene in a close proximity with T-cell specific transcriptionally active loci within nucleus is essential for aberrantly activating TAL1 oncogene in T-ALL. In this proposal, we will investigate the underlying molecular mechanisms that connect chromatin loops with transcriptional activation decision of the TAL1 oncogene as well as elucidate the role of CTCF and enhancer regulatory elements mediated chromatin interactions in regulation of TAL1 gene during normal hematopoiesis and leukemogenesis. The specific aims are: 1) Evaluate the role of CTCF mediated genome organization in regulation of enhancer/ promoter interaction and TAL1 transcription during hematopoiesis and T cell leukemogenesis; 2) Study the molecular mechanisms by which interchromosomal loops result in aberrant activation of TAL1 oncogene in T-ALL.